(1) Field of the Invention
The present invention relates to an optical waveguide substrate and an optical wave guide device using the above optical waveguide substrate which is used preferably in conjunction with high speed optical modulators optical switching devices, a second harmonic generation device. The invention also relates to a process of producing the optical waveguide substrate.
(2) Related Art Statement
The ridge type optical waveguide is used as part of optical modulators, optical switching devices and so on. In the field of the high speed optical modulators and switching devices for optical communications, a modulator or a switching device produced by forming an optical waveguide element by diffusing titanium into lithium niobate single crystal, which can modulate or switch optical signals with a high speed more than 10 GHz, is known. Moreover, as a light source for blue laser preferably used for an optical pick-up and so on, a second harmonic generation (SHG) device of quasi phase matched (QPM) type, in which a periodic polarization inversion construction is formed in lithium niobate single crystal or lithium tantalate single crystal and then an optical waveguide portion is formed by using a proton exchange method, is also known. The second harmonic generation device mentioned above can be widely applied to devices for optical disk memory, medical use, photochemical use, optical measurement use, and so on.
Generally, as a method of forming an optical waveguide having a ridge construction, a method of transferring a mask pattern on a substrate by means of a photolithography technique and removing a portion other than the mask pattern by means of an ion etching is known. In the case of an optical modulator for modulating intensity, phase, wavelength and so on of a light, in which electrodes for applying an alternating electric field for modulating are arranged with respect to the ridge type optical waveguide, it is theoretically known that, if a ridge angle becomes near to 90.degree. more and more, an applied electric field correction factor is increased correspondingly and a driving voltage can be reduced (Japanese Patent Laid-Open Publication No. 4-123018). In the JP-A-4-123018 mentioned above, a ratio of d/W in the ridge type optical waveguide between a height d and a width W of the ridge portion is set to larger than 0.1 and smaller than 1.0, and a ridge angle of the ridge portion is set to 90.degree..+-.10.degree., so that the applied electric field correction factor increases as large as possible.
However, according to this known method in which a portion other than the mask pattern is removed by using the ion etching, it is difficult to make a ridge angle 90.degree. for the following reason. As shown in FIGS. 10a and 10b, in the known method, an epitaxial layer 27 is formed on a substrate 26 made of electrooptical single crystal, and a mask 28 having a predetermined pattern is formed on a main plane 27a of the epitaxial layer 27 by using a photolithography method. Then, the epitaxial 27 is etched to form a ridge type optical waveguide 31. In this case, since a portion of the epitaxial layer 31a other than a portion covered by the mask 28 is removed, a thickness of an epitaxial layer 30 formed by etching the epitaxial layer 27 becomes smaller, as shown in FIG. 10b. Up to now, the known ridge type optical waveguide 31 having a ratio of d/W larger than 1.0 has not been achieved.
As a result an improvement of the applied electric field correction factor is limited. That is, in the known optical modulator, a surface 31a of the ridge type optical waveguide 31 is substantially flat, and a side surface 31b is inclined. Moreover, an electrode layer 32 for modulation is formed on a main plane 30a of the epitaxial layer 30 and on the inclined side surface 31b. Therefore, as discussed in "RIDGE TYPE OPTICAL WAVEGUIDE", OQE77-57, Oct. 24, 1997, Institute of Electronics, Information and Communication Engineers, if an alternating electric field for modulation is applied to the optical waveguide 31, the modulation efficiency using the alternating electric field is decreased in comparison to an optical waveguide in which a ridge angle is 90.degree., thereby increasing driving voltage.
The side surface 31b is inclined for the following reasons. In the known optical modulator, the ridge type optical waveguide 31 is projected from the main plane 30a of the epitaxial layer 30. In this case, in order to increase a ratio d/W between a height d and a width W of the ridge type optical waveguide 31 (i.e., in order to make the ridge type optical waveguide 31 thinner), it is necessary to etch a portion around the ridge type optical waveguide 31 as deep as possible. However, since a ratio of etching rate between a substrate side portion (epitaxial layer 27 and substrate 26) and the mask 28 is 2.5(substrate side portion):1(mask), it is necessary to use a mask having a large thickness to allow for deep etching of the portion around the ridge type optical waveguide 31. Since the mask 28 has a large thickness, the etching liquid is not sufficiently supplied near the mask 28. Thus an etching rate of the portion near the mask 28 is decreased, so that a ridge angle (.theta.) is extremely decreased from 90.degree.. For example, if a height d of the ridge type optical waveguide 31 is set to larger than 2 .mu.m, it is difficult to achieve a ridge angle (.theta.) of 90.degree..
Japanese Patent Laid-Open Publication No. 4-123018 aims at solving these problems mentioned above, but the fundamental defect explained with reference to FIGS. 10a and 10b is not solved at all. This publication only limits a dimension of the known ridge type optical waveguide.
Moreover, the ridge type optical waveguide substrate shown in FIGS. 10a and 10b is used generally for an optical coupling with an end surface of optical fibers. In this case, it is necessary to reduce a coupling loss as much as possible, and also it is necessary to reduce a transmission loss in the optical waveguide. Further, in the case that a single mode light is incident upon the ridge type optical waveguide, it is necessary to improve an extinction ratio more and more.